Engine speed control via the torque converter lockup clutch of a continuously variable drive

ABSTRACT

Within the scope of the method for controlling the engine rotational speed via the torque converter lock-up clutch during the starting and stopping phases of a motor vehicle having a continuously variable transmission combined with other elements of the drive train, the closing and opening of the torque converter lock-up clutch are used as implicit function so as to ensure optimum comfort via the most asymptotic adjustment possible of the rotational speeds gradients of engine and turbine during starting and by a soft, defined separation of the speed characteristic curves of the engine and turbine during stopping. When the vehicle is standing still the forward clutch is moved into standstill disengaged mode so that the torque converter lock-up clutch can be fully closed.

FIELD OF THE INVENTION

This invention relates to a method for engine speed control via thetorque converter lockup clutch combined with other elements of the drivetrain of a continuously variable transmission.

BACKGROUND OF THE INVENTION

Hydrodynamic torque converters in automatic transmission perform thetask of ensuring a comfortable starting operation with sufficientstarting power. Besides, by means of the hydrodynamic torque converterthe torsional vibrations of the engine are uncoupled form the drivetrain. The efficiency losses resulting form the use of a hydrodynamictorque converter are reduced by the torque converter lockup clutch.

In the conventional method for control of the torque converter lockupclutch in stepped and continuously variable automatic transmissions, thetorque converter is closed when starting via characteristic linesdependent on rotational speed. Subject to the differences of torquebetween input and output side, the torque converter is here “wound up”at the beginning of the starting operation, which generally results inthe generation of a significant rotational speed difference betweenengine rotational speed and turbine rotational speed. Said difference inrotational speed depends on the torque converter characteristic(soft/hard) and can be distinctly above 1000 revolutions per minute.

In the customary closing of the torque converter lockup clutchcontrolled by characteristic lines there generates, specially in thelower load range and subject to the generally lower nominal operatingpoint, a clearly negative rotational speed gradient of the engine. Theclosing of a torque converter lockup clutch is then felt by the driveras an extremely disturbing overshoot, which also adds negatively to theexperience.

Furthermore, in a conventional transmission controlled by characteristiclines there generates in the coasting operation, when opening the torqueconverter lockup clutch, the usual “dipping” of the engine rotationalspeed below the turbine rotational speed, a fact that the drivergenerally will find disturbing.

The Applicant's DE 196 31 071 A1 discloses a control system for acontinuously adjustable transmission. The engagement or disengagement ofthe torque converter lockup clutch (WK) is here implemented as functionof the control of the nominal rotational speed of the engine. Thismethod makes it possible to reduce the overshoot of the enginerotational speed when starting.

However, the overshoot of the engine rotational speed cannot be entirelydominated thereby: at the start of the regulating phase a distinctincrease of the engine rotational speed is to be detected. The cause ofsaid effect is the reaction times of the torque converter to an outercontrol which amount to a few hundred milliseconds (msec), since the oilrearrangement operation (reversal WK at operation to WK to operation)have to be carried out first.

Only after the oil rearrangement operations have been finished in thetorque converter can the torque converter lockup clutch build up thetorque needed for control of the engine rotational speed along thetrajectory. In an assumed time constant of 40 msec for build up andbreakdown of pressure, about 200 msec lapse in this example until thetorque converter lockup clutch receives the desired direct connection.The normal average reaction times still are clearly above that. Duringthis time there is no influence on the behavior of the torque converterlockup clutch so that also no control of the engine rotational speed ispossible according to this application.

In conventional transmissions of the prior art different circumstancescan cause the torque converter lockup clutch not to be fully openwhereby a down of the engine is caused when stopping.

Therefore, this invention is based on the problem of providing,departing from the cited prior art, a method for control of the enginespeed via the torque converter lockup clutch, which method ensuresduring the starting phase an immediate reaction of the torque converterlockup clutch and thus optimum comfort with enough starting power, andin the stopping phase prevents “stalling” of the engine resulting from amalfunction of the torque converter lockup clutch and the lowering ofthe engine rotational speed below the turbine rotational speed incoasting operation.

In particular the above mentioned overshoot of the engine rotationalspeed when starting must be reduced as far as possible.

In addition the inventive method must be of reasonable cost and easy toimplement.

SUMMARY OF THE INVENTION

It is accordingly proposed to use the closing and opening of the torqueconverter lockup clutch as an implicit function so as to ensure anoptimum comfort by the most asymptotic adjustment possible of therotational speed gradients of engine and turbine when starting and by asoft, defined separation of the rotational speed curves of engine andturbine when stopping, and thereby to minimize the reaction times of thetorque converter by moving the forward clutch while the vehicle standsstill to the standstill disengaged mode so that the torque converterlockup clutch can be fully closed.

When starting the engine rotational speed is here approximated by thetorque converter lock-up clutch to a defined trajectory, the enginerotational speed being equal to the turbine rotational speed at the endof the trajectory. Consequently, an overshoot of the engine rotationalspeed when starting is reduced and a jolt-free comfortable closing ofthe torque converter lock-up clutch is ensured.

According to this invention it is provided that the overshoot of theengine rotational speed at the beginning of the regulating phase whenstarting be additionally reduced by minimizing the reaction times of thetorque converter to an outer control, which can amount to severalhundred msec.

Only after the oil rearrangement operations have been finished in thetorque converter can the torque converter lock-up clutch build up thetorque needed for control of the engine rotational speed along thetrajectory. For the earliest adaptation of the engine speed during thestarting mode, it will be necessary for the converter lock-up clutch toinitiate a direct connection to the system immediately at the beginningof the control phase. The invention suggests to conclude thetime-consuming oil transferring processes in the torque converter asearly as possible and to keep the torque converter lock-up clutch inactive stand-by mode.

This is achieved within the scope of this invention by the fact that thetorque converter lock-up clutch is operated when the vehicle standsstill in interplay with the forward clutch.

At the same time, the forward clutch must be operated in “standstilldisengaged mode” (SBC standby control) which, in this range, makes itpossible to fully close the torque converter lockup clutch.

In the SBC operation known from the prior art, while the vehicle isstopped, the forward clutch (in position “R” applies also to thecorresponding shifting element) is opened precisely to the extent thatthe input torque needed for moving off is in the load-free stateprecisely somewhat below the outcropping, load torque. As a result isprevented in operation in position “D” without actuating the vehiclebrake the moving off typical for automatic transmission. By this step inaddition to the efficiency losses are reduced due to movement of theoperating point of the torque converter. Therefore, the SBC functionalso brings with it a reduction in consumption.

The power loss and therewith the heat load produced on the torqueconverter lock-up clutch are hereby outside the critical range.

This procedure eliminates the shifting characteristic lines and thethreshold value questions for closing and opening the torque converterlockup clutch.

The defined control of the engine used within the scope of thisinvention has an advantage in that the compromise between starting powerand rotational speed overshoot essential for a positive driving feelingcan be implemented in an optimal way.

Besides, the inventive method results in a reduction of fuelconsumption.

To ensure when stopping a reliable opening of the torque converterlock-up clutch, there are not admitted, according to the invention,engine rotational speed values below a defined threshold value over theidling speed. It is additionally provided within the scope of thisinvention to introduce an early opening of the torque converter clutchby means of a boosting of the nominal rotational speed of the engine orincrease of the nominal ratio value.

It is further proposed entirely to prevent, by a defined enginerotational speed standard and control of the torque converter lock-upclutch, the “dipping” of the engine rotational speed below the turbinerotational speed usual in coasting operations, which the drivergenerally senses as disturbing. To that end the engine nominalrotational speed is kept as long as possible at the turbine nominalrotational speed. If as a result of disturbing marginal effects (such asresonance humming on account of the almost closed torque converterlock-up clutch in the lower rotational speed range) it is not possibleto keep the engine rotational speed value above the turbine rotationalspeed only by control of the torque converter lock-up clutch, it is alsopossible here to supportively raise the ratio value.

According to the invention, when the torque converter lock-up clutchcannot be fully opened, the engine stalls when stopping by the fact thatthe drive train is interrupted upon detection of this malfunction of thetorque converter lock-up clutch.

In this case the engine rotational speed trajectory, which is normallyused to control the torque converter lock-up clutch, serves as standardvalue for the slip regulation of the forward clutch.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention is explained in detail herebelow with reference to thedrawings which show:

FIG. 1 a fundamental illustration of a starting and stopping operationaccording to a conventional method; and

FIG. 2 a fundamental illustration of a starting and stopping operationby the method according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 show a closing operation of the torque converter lock-up clutchaccording to the prior art. The line n_mot shows the engine rotationalspeed and the line n_t shows the rotational speed curve of the turbine.The overshoot of the engine rotational speed, when starting, is clearlydetected. This range is shown as hatching. The rotational speeds of theengine and of the turbine are adjusted only after the torque converterclutch is fully closed.

This overshoot of the engine rotational speed, during the startingphase, is extremely disturbing to the driver.

In FIG. 1, the usual dipping effect of the prior art of the enginerotational speed below the turbine rotational speed in the coastingoperation when opening the torque converter lock-up clutch is alsoillustrated. The driver also senses this effect as generally disturbing.

FIG. 2 illustrates the starting and stopping operations according tothis invention.

The dotted line is the trajectory n_mot_soll of the nominal rotationalspeed of the engine.

The nominal value for the trajectory of the engine rotational speed isdeduced from an actual operating point standard for the ratio accordingto the equation

n _(—) mot _(—) soll=iv _(—) soll·n _(—) s 2

wherein n_mot_soll is the nominal rotational speed of the engine,iv_soll the nominal ratio, and n_s2 the rotational speed of a secondarydisc.

An abrupt increase in the engine rotational speed, at the beginning ofthe regulating phase, is prevented by minimizing the reaction time ofthe torque converter by the inventive preconditioning in standstill.

The preconditioning consists in that time-consuming oil rearrangementoperations, in the torque converter, are terminated as early as possiblewhereby the torque converter lock-up clutch is maintained ready foraction.

This occurs by moving the forward clutch to a SBC operation. Thereby afull closing of the torque converter lock-up clutch is possible.

Within the scope of the inventive method, the torque converter lock-upclutch, closed after passage of the forward clutch to the SBC operation,is driven from the control pressure level to near its slip limit inorder to be opened as quickly as possible in case of a sudden startingoperation for which the forward clutch must be engaged. The influenceupon the engine rotational speed is accordingly effective and theinterfering overshoot can be almost entirely compensated.

The engagement of the forward clutch and the passage of the torqueconverter lock-up clutch to the engine rotational speed controlcomplementarily develop by means of an overlapping gear shift.

At the same time, a superimposed driving strategy ensures that theengine rotational speed, at the closing of the torque converter lock-upclutch, is inverted to the turbine rotational speed.

From FIG. 2 it becomes clear that the engine rotational speed level isdistinctly below that of the closing operation controlled by shiftcharacteristic lines of the torque converter lock-up clutch. The effectof the increase of the engine rotational speed can be almost fullyprevented. Likewise clear is the soft adjustment of the rotationalspeeds of engine and turbine during closing of the torque converterlock-up clutch according to the invention.

As reliable as possible an opening of the torque converter lock-upclutch when stopping is ensured by the fact that the engine rotationalspeed nominal values below a defined threshold value over the idlingspeed are not admitted. For assistance the driving strategy can inaddition correctingly interfere by boosting the nominal rotational speedof the engine above the ratio standard in order to introduce an earlyopening of the torque converter lock-up clutch.

The “dipping” of the engine rotational speed below the turbine.rotational speed, usually during a coasting operation when opening thetorque converter lock-up clutch, is entirely prevented—as clearlyillustrated in FIG. 2—by the defined engine rotational speed standardand control of the torque converter lock-up clutch. According to theinvention, the engine nominal rotational speed is kept at the turbinerotational speed value as long as possible.

In case of a malfunction when opening the torque converter lock-upclutch, the necessary interruption of the drive train occurs via theforward clutch, the engine rotational speed trajectory serving asstandard value of the slip regulation of the forward clutch.

As soon as the vehicle reaches a standstill, the engine rotational speedcontrol function of the forward clutch passes to the SBC operation. Ifsaid operating state has been definitely reached, the torque converterlock-up clutch is passed to the above described closed operation on theslip limit. Consequently, during standstill the same operating state offorward clutch and torque converter lock-up clutch always exists; thisoccurs independently of whether the stopping has been carried out withtorque converter lock-up clutch opening, without any problem, or onlyusing the proposed function via the forward clutch.

In the case of the torque converter lock-up clutch suddenly pullingapart during the substitute function via the forward clutch, the forwardclutch converts to a “closed” state. Here whether the torque converterlock-up clutch remains open is continuously monitored. If standstill ofthe vehicle is again detected, then the passage of the forward clutch tothe standstill disengaged mode occurs in open state of the torqueconverter lock-up clutch and thereafter the torque converter lock-upclutch is again passed to the closed operation on the slip limit.

If prior to reaching a standstill of the vehicle, the driver again stepson the gas (starting through), the forward clutch is first closed andthereafter the torque converter lock-up clutch is closed, the torqueconverter lock-up clutch is again passed to the preset rotational speedtrajectory of the engine, as described already.

A specially advantageous variant of this invention provides that inorder to prevent a thermal overload of the torque converter lock-upclutch (the same as of the whole torque converter) the actual power lossbe continuously calculated during the entire “slip operation” andcompared with a preset admissible limiting value. When exceeding saidlimiting value, the torque converter lock-up clutch is opened in orderto effect a cooling by the now increased flow through the torqueconverter. If the difference between engine and turbine rotationalspeeds has already decreased to a very small value (such as 50-100 rpm),when exceeding the limiting value, the torque converter lock-up clutchcan be entirely closed so as to forestall further heat feeding.

Only after lapse of a locking time adapted to the cooling process is thetorque converter lock-up clutch again released for the slip operationand thus for control of the engine rotational speed.

According to the invention, the thermal load limits are monitored by aphysico-mathematical patter.

When circumstances demand it, the inventive functions of the forwardclutch are taken over by the reverse clutch.

What is claimed is:
 1. A method for engine rotational speed control viaa torque converter lock-up clutch during the starting and stoppingphases of a motor vehicle having a continuously variable transmissioncombined with other elements of the drive train, the method comprisingthe steps of: using the closing and opening of the torque converterlock-up clutch as an implicit function defined by a most asymptoticadjustment as possible of the rotational speed gradients of the engineand a turbine when starting and by soft, defined separation between anengine rotational speed curve and a turbine rotational speed curve whenstopping, and wherein an optimum comfort is ensured in that when thevehicle stands still a forward clutch is moved to a standstilldisengaged mode so that the torque converter lock-up clutch can be fullyclosed.
 2. The method according to claim 1, wherein at starting theengine rotational speed is approximated by the torque converter lock-upclutch to a defined trajectory, the engine rotational speed at the endof the trajectory being the same as the turbine rotational speed.
 3. Themethod according to claim 2, wherein the trajectory is defined accordingto n _(—) mot _(—) soll−iv _(—) soll·n _(—) s 2 n_mot_soll being thenominal engine rotational speed, iv_soll the nominal ratio and n_s2 therotational speed of the secondary disc.
 4. The method according to claim3, wherein a pressure level of the closed torque converter lock-upclutch is driven closely above a pressure value where a slip-freetransfer of the outcropping engine torque is ensured so that thereaction times be minimized when passing to the engine rotational speedcontrol operation at starting.
 5. The method according to claim 4,wherein the closing of the forward clutch and the passage of the torqueconverter lock-up clutch to the engine rotational speed controloperation run complementarily by using an overlapping gear shift.
 6. Themethod according to claim 1, wherein during coasting operation of thevehicle a lowering of the engine rotational speed below the turbinerotational speed when opening the torque converter lock-up clutch isprevented by an engine rotational speed standard defined by the drivingstrategy and control of the torque converter lock-up clutch.
 7. Themethod according to claim 6, wherein the engine rotational speedstandard is kept at the turbine rotational speed value until shortlybefore reaching an engine idling speed.
 8. The method according to claim7, wherein starting from an adjustable moment shortly prior to reachingthe engine idling speed, the engine rotational speed standard isconverted to a defined value above the engine idling speed.
 9. Themethod according to claim 1, wherein the drive train is interrupted upondetection of a malfunction of the torque converter lock-up clutch viathe forward clutch, the engine rotational speed trajectory serving as apreset value for slip regulation of the forward clutch.
 10. The methodaccording to claim 9, wherein while the vehicle stands still the slipregulation function of the forward clutch passes to the standstilldisengaged mode so that in standstill there is always the same operatingmode of the forward clutch and torque converter lock-up clutch.
 11. Themethod according to claim 9, wherein in case of sudden opening of thetorque converter lock-up clutch, the forward clutch is closed, therebeing continuously monitored whether the torque converter lock-up clutchremains in open state.
 12. The method according to claim 1, whereinduring the engine rotational speed control operation of the torqueconverter lock-up clutch the power loss is continuously calculated andcompared with a preset limiting value.
 13. The method according to claim12, wherein when the limiting value is exceeded the torque converterlock-up clutch is opened.
 14. The method according to claim 13, whereinafter lapse of a present locking time which takes into account a coolingoperation, the torque converter lock-up clutch is again moved to theengine rotational speed control operation.
 15. The method according toclaim 12, wherein when the difference between engine rotational speedand turbine rotational speed is reduced to a value smaller than thepreset limiting value, the torque converter lock-up clutch is fullyclosed.
 16. A method for controlling engine rotational speed via atorque converter lock-up clutch during the starting and stopping phasesof a motor vehicle having a continuously variable transmission, themethod comprising the steps of: starting the vehicle by disengaging aforward clutch and fully closing the torque converter lock-up clutchwhile the vehicle is in a standstill position to ensure an asymptoticadjustment between an engine rotational speed and a turbine rotationalspeed; and stopping the vehicle by opening the converter lock-up clutchand ensuring that the engine rotational speed does not fall below theturbine speed and substantially equals the turbine rotational speed foras long as possible to provide a moderate defined separation between theengine rotational speed and the turbine rotational speed when stopping.17. The method according to claim 16, further comprising the steps ofduring starting of the vehicle operating the torque converter lock-upclutch according to a theoretical engine rotational speed function andadjusting the engine, rotational speed, to the theoretical enginerotational speed function until the engine rotational speed is the sameas the turbine rotational speed.
 18. The method according to claim 17,further comprising the step of determining the theoretical enginerotational speed function according to n _(—) mot _(—) soil−iv _(—)soll·n _(—) s 2 n_mot_soll being the nominal engine rotational speed,iv_soll the nominal ratio and n_s2 the rotational speed of the secondarydisc.
 19. The method according to claim 18, further comprising the stepof ensuring a pressure level of the closed torque converter lock-upclutch remains closely above a pressure value for a slip-free transferof the engine torque so that reaction times can be minimized during theengine rotational speed control operation at starting.
 20. A method forengine rotational speed control via a torque converter lock-up clutchduring the starting and stopping phases of a motor vehicle having acontinuously variable transmission combined with other elements of adrive train, the method, comprising the steps of: using the closing andopening of the torque converter lock-up clutch as an implicit functiondefined by a most asymptotic adjustment as possible of the rotationalspeed gradients of the engine and a turbine when starting and by soft,defined separation of the rotational speed curves of the engine and theturbine when stopping, and an optimum comfort is ensured in that whenthe vehicle stands still a forward clutch is moved to a standstilldisengaged mode so that the torque converter lock-up clutch can be fullyclosed; wherein at starting the engine rotational speed is approximatedby the torque converter lock-up clutch to a defined trajectory, theengine rotational speed at the end of the trajectory being the same asthe turbine rotational speed.